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Title

A method for estimating groundwater contribution areas for Illinois nature preserves and other natural areas.

Description

A new method for estimating the groundwater contribution area (GCA) for Illinois nature preserves was demonstrated using 12 test sites (Bluff Springs Fen, Braidwood Dunes and Savanna, Elizabeth Lake, George B. Fell, Goose Lake, Illinois Beach, Lake in the Hills Fen, Lockport Prairie, Parker Fen, Romeoville Prairie, Spring Grove Fen, and Volo Bog). The sites were selected for their varied hydrogeologic settings and available hydrogeologic data. None of the sites had sufficient local groundwater studies available to identify an entire GCA. Regional studies available for six preserves readily could be used to identify a regional GCA. The amount of hydrogeologic data available for any given preserve will vary, but for most Illinois nature preserves groundwater studies are not available to evaluate groundwater flow conditions. Because contribution areas must be determined to address site management issues, this new method accommodates those sites by identifying GCAs using available information. In particular, it uses published hydrologic and geologic data, if available, as well as uncompiled water-level data, and proxy data adjusted by best professional judgment to account for significant features affecting shallow, unconfined groundwater flow. Surface watersheds were delineated and adjusted based on significant hydrologic features (e.g., water elevations in ponds, streams, and wetlands; infrastructure such as ditches, sewers, and roadways) to develop adjusted surface watershed areas (ASWAs) for all 12 sites. These ASWAs and regional GCAs were compared to determine the viability of substituting ASWAs for regional GCAs at preserves lacking groundwater-level data. The ASWAs identified between 7 and 68 percent of the regional GCAs. More importantly, the ASWAs included the most hydrologically significant locations directly upgradient of each preserve. Use of an ASWA to estimate groundwater flow will not be effective in some hydrogeologic settings, including those where confined groundwater sources, karst terrains, and significant groundwater withdrawals may be a factor. Regional GCAs and ASWAs were combined at each site to create final GCAs that attempt to identify all areas that could contribute groundwater to a nature preserve. Final GCA estimates presented in this report will be useful to determine areas where a Class III (also known as Special Resource) groundwater designation could be applied under Title 35 Section 620.230(b) of the Illinois Administrative Code. They also identify areas of important groundwater resources where it may be most effective to focus management or acquisition efforts to ensure preserve integrity.